ION Publications

Characterization of the Galileo Ranging Accuracy and Integrity Performance: Methodologies and Results

With the declaration of the Initial Services in December 2016, Galileo entered into the exploitation and operation phase: multi-constellation receivers can exploit the advantages of using Galileo, either in combination with other constellations or autonomously. Its use could be extended also to new applications as the studies in the standardization bodies at international and European level show.
In the civil aviation framework, the European Organisation for Civil Aviation Equipment (EUROCAE) is preparing a Minimum Operational Performance Standard (MOPS) for Galileo, GPS and SBAS airborne receivers for a dual-constellation (GPS+Galileo) dual-frequency (L1/E1+L5/E5a) receiver. The European Rail Transportation Management System (ERTMS) stakeholders took recently the decision to adopt GNSS for improving rail transportation and benefit from existing GNSS infrastructures. Significant stringent requirements are also imposed in the automotive sector by emerging connected cars and automated driving technologies. Similarly, UAS Traffic Management (UTM), enabling civilian low-altitude airspace and UAS operations, is asking for high integrity location services supporting dynamic geofencing, terrain avoidance, route planning and re-routing.
In this phase it is essential to monitor the system and properly characterize the user performance. At the same time, anomalies need to be promptly and reliably detected in particular for safety critical applications with high demanding integrity requirements. With the above background, DLR has implemented a routine monitoring of Galileo Key Performance Indicators that complements similar activities by the System Provider and Operator, and may assist their independent validation.
This paper describes this activity since March 2016. Its main scope is the evaluation of the performance from the user point of view by using public data, mostly made available by the International GNSS Service (IGS) and its Multi GNSS Experiment (MGEX).
In the paper the methodologies used to evaluate the performance are presented, discussed and motivated. Slight differences in the assumptions, algorithms and even in the interpretation of the KPIs can lead to significantly different results and need to be described. Furthermore, DLR's performance monitoring has no access to system parameters (e.g. antenna phase center offsets, satellite antenna pattern, differential code biases …) which are used as part of the actual Galileo system operations. These aspects are described and analyzed in the paper.
The analysis focuses on the Open Service for dual and single frequency users and covers the satellite orbit and clock errors, the signal-in-space availability, the positioning accuracy, the ranging bounding parameters, the integrity risk and the continuity risk.
The Galileo satellite orbit errors are evaluated for the F-NAV messages on E5a frequency and for the I-NAV message on E1 and E5b frequencies. The broadcast ephemerides are generated from real-time streams of about 30 IGS multi-GNSS stations. Precise orbit and clock parameters as well as differential code biases are also estimated by DLR.
The Signal In Space Ranging Error (SISRE) as 95-percentile in nominal condition is described and selected anomalies are identified. Outlier's exclusion approaches are used in order to assess nominal performance also in presence of anomalies.
The satellite clock stability is analyzed using various GNSS stations connected to Hydrogen masers and some to the UTC network. The clock error is evaluated over arcs of 3 days based on the overlapping Allan deviation
The second part of the paper focuses on the user performance in the position domain with a particular focus on future integrity service for aviation and other applications. Signal-in-space parameters which are relevant for the Advanced RAIM concept and the generation of the Integrity Support Message are monitored and analyzed. To this purpose the paper focuses on two aspects, for which novel monitoring methodologies are described and used.
First of all the bounding of the ranging error is addressed. Several bounding definitions and methods can be used for the generation of the User Range Accuracy (URA) sigma provided in the Integrity Support Message (ISM) [1,2, 3]. Each of them solves differently the problem of assessing statistic characteristics of the SISRE distribution tails with a limited sample size. The strict aviation integrity requirements (even stricter for rail applications) require extrapolation strategies in the ground monitoring. On the other side the ARAIM ground monitoring can take advantage of the fact that it has to perform a bounding monitoring rather than a bounding estimation: the broadcast URA/SISA needs to be verified and if the observations show evidence of a bounding violation, the ISM sigma needs to be increased [6]. Performing a monitoring instead of a bounding estimation allows reaching confidence on higher percentiles with smaller sample size. This method will be used on the real Galileo data and results will be presented and compared to state of art techniques [1, 2, 3].
Second the paper discusses the continuity and integrity risk of the user. In fact integrity and continuity requirements have been up to now tailored to the aviation user needs. In this context the risks are interpreted in an average sense, by computing probabilities of events over a certain period of time and scaling them to the duration of the specific operation [4, 5]. These approaches don't take into account that the continuity risk has per definition an evolution over time: it is the probability of having the system continuously maintained available. Continuity can be lost in a certain instant of time only if it was not lost in advance. These approaches [4,5] are valid for aviation phase of flights which are defined on short duration (up to 150s). But the extension of ARAIM to other applications (rail, automotive, UAVs) with longer operation durations and higher level of criticism of the continuity requirements need more accurate methods. The paper presents a Markov model for the computation of the continuity risk [7]: each satellite health status is modelled with a Markov process using the GPS Mean Time Between Failures (MTBF) and the Mean Time To Repair (MTTR). The user continuity risk resulting from the ARAIM Fault Detection and Exclusion algorithm [8] is then computed propagating over time the user healthy status. The paper compares this algorithm [7] to state of art techniques [4,5].
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[1] Rife, J. el al, "Overbounding SBAS and GBAS Error Distributions with Excess-Mass Functions", GNSS 2004 The 2004 International Symposium on GNSS/GPS Sydney, Australia 6–8 December 2004
[2] Rife, J. el al, "Paired Overbounding for Nonideal LAAS and WAAS Error Distributions", IEEE Transaction of Aerospace and Electronic System, Vol. 42, No. 4, 2006
[3] Rife, J. el al, “Core Overbounding and its Implications for LAAS Integrity,” Proc. ION GNSS 2004, Long Beach, CA, Sept. 21-24, 2004, pp. 2810-2821.
[4] Joerger, M. el al, "Fault Detection and Exclusion Using Solution Separation and Chi-Squared ARAIM ", IEEE Transaction of Aerospace and Electronic System Vol. 52, No. 2 2016
[5] Joerger, M. el al, " Integrity Risk Minimisation in RAIM Part 2: Optimal Estimator Design", The Journal of Navigation, Vol. 69, pp. 709–728, 2016 doi:10.1017/S0373463315000995
[6] Martini, I. el al, "Integrity Support Message Architecture Design for Advanced Receiver Autonomous Integrity Monitoring", Proc. European Navigation Conference 2013, 23-25 April 2013, Vienna, Austria
[7] Franzese, G. el al, "A novel Markov Model for the computation of the continuity risk in Maritime applications", Proc. ION ITM 2017, Jan 30-Feb 4, Monterey, CA, pp. 226-247, 2017.
[8] Working Group C, ARAIM Technical Subgroup, EU-US Cooperation in Satellite Navigation. “Milestone 3 Report”. March 2016. http://www.gps.gov/policy/cooperation/europe/2016/working-group-c/ARAIM-milestone-3-report.pdf

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Proceedings of the 30th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2017)
September 25 - 29, 2017
Oregon Convention Center Portland, Oregon